Immunotoxin derived from a recombinant human autoantibody and method of using thereof

ABSTRACT

The invention is directed to an immunotoxin directed at fetal AchR, wherein the immunotoxin may comprises a human Fab fragment based on a human autoantibody or human combinatorial cDNA library and may be a pseudomonas exotoxin A-based single chain Fv IT (35-scFV-ETA). The invention is further directed to method of treating a patient with soft tissue tumour comprising the step of exposing the patient to the immunotoxin of the invention.

CROSS REFERENCE DATA

This application claims the priority of Provisional U.S. Application No. 60/507,131 filed Oct. 1, 2003.

BACKGROUND OF THE INVENTION

Rhabdomyosarcomas (RMS), according to the WHO classification, comprise three types of tumours sharing skeletal muscle differentiation while showing distinct genetic and biological features: 1) embryonal with a characteristic loss of heterozygocity at 11p15; 2) the more aggressive alveolar, most of which show pathognomonic Pax3 or Pax7-FKHR fusion genes due to the translocations t(2;13)(q35;q14) and t(1;13)(p36;q14), respectively; and 3) the rare pleomorphic RMS with complex genetic aberrations and preferential occurrence in adults. As a group, RMS are the most frequent soft tissue tumour in children and have a poor prognosis. In particular, metastatic alveolar RMS in children more than 10 years old are often refractory to all current therapies including adjuvant bone marrow transplantation, resulting in 5-year survival rates of 5-20%. Immunotherapies or other (e.g. genetic) specific targeting strategies have not yet been applied due to the absence of a well defined RMS-specific gene target and the lack of suitable immunotherapeutic tools. However, it was shown previously that strong expression of fetal AChR mRNA and protein is virtually pathognomonic for non-innervated rhabdomyoblasts and rhabdomyotubes and, thus, for the vast majority of human embryonal and alveolar RMS; by contrast, fetal AChR expression is virtually absent in other tissues and in tumours without a rhabdomyoblast component.

The nicotinic AChR of skeletal muscle is a pentameric ion channel composed of five subunits. The fetal isoform is composed of 2α, 1β, 1γ ad 1δ-subunits, while the adult isoform exhibits an ε-subunit instead of the γ-subunit. During the later stages of development in uterus, the fetal AChR is gradually replaced by the adult isoform in virtually all innervated muscles. After birth, strong expression of fetal AChR is restricted to a few muscle fibres of unknown function occurring in extraoccular muscles and myoid cells of the thymus that are physiologically non-innervated muscle cells. However, high-level re-expression of fetal AChR can occur after birth following denervation, including denervating neuromuscular diseases.

In adult human innervated skeletal muscles, therefore, fetal AChR expression is exceedingly low. Only traces of γ-subunit mRNA can be detected by RT-PCR and RNAse protection assay, and very little by Northern blot. In addition, it has been shown that the mothers of newborns with arthrogryposis multiplex caused by the placental transfer of maternal antibodies specific to fetal AchR can be completely asymptomatic, suggesting that there is minimal expression of AChR γ-subunit protein on normal adult muscle.

Immunotoxins (ITs) are usually composed of a plant or bacterial toxin (like Pseudomonas exotoxin A=ETA) coupled to an antibody fragment for specific targeting of cells in the context of cancer, chronic inflammatory diseases, including autoimmune diseases and chronic infections, or transplantation. The antibody component of ITs is most often a murine Fab or Fv antibody fragment that has been humanised to minimize sensitization against the xenogeneic protein. Immunotoxins with a fully human antibody moiety (directed to HIV infected T-cells) have also been reported.

Impressive therapeutic efficacy of ITs has been demonstrated in a number of lymphomas and leukemias following short term application of the IT. By contrast, protracted use of an IT might be more adequate in poorly vascularized sarcomas. In such a clinical setting, is probably essential to minimize the sensitization against the mouse, rat or humanized antibody component of the IT.

Autoimmune disorders collectively cause much suffering and disability and many are particularly common in women of child-bearing age. The muscle weakness in the majority of myasthenia gravis (MG) patients results from autoantibodies to the acetylcholine receptor (AChR) which cause AChR loss, mainly by complement-mediated damage to the post-synaptic membrane and/or accelerated AChR degradation. The AChR consists of two α subunits and one β, γ and δ in the fetus. Together, these subunits form a cation-specific ion channel that opens when ACh binds simultaneously to the two sites at the α/δ and either the α/γ or α/ε interfaces. From about 30 weeks' gestation in humans, the ‘adult’-specific ε subunit gradually replaces the γ subunit, but the fetal isoform continues to be expressed on rare myoid cells in the adult thymic medulla.

Early-onset MG (before age 40; EOMG) shows a 3:1 female bias and a strong HLA-DR3-B8 association. The thymus is apparently an important site of autoimmunization; thymectomy is often beneficial and, in most EOMG cases, the thymus is invaded by lymph node-like T cell areas and germinal centers (GC, many of which show AChR-specificity. GC are well known sites of B cell memory generation and of antibody diversification by antigen-selection of somatic variants. Moreover, there is selective activation of thymic plasma cells spontaneously producing anti-AChR antibodies, with similar specificities to those in the patients' blood. The anti-AChR antibodies in typical MG patients are very heterogeneous and antibodies cloned from the thymus of typical EOMG patients show a range of specificities resembling those in the donors' sera.

About 8% of MG mothers have babies with transient neonatal MG, and their sera tend to have higher antibody titers against fetal than adult AChR, consistent with a role for thymic AChR in inducing their disease. In some rare examples, babies of MG mothers are born with severe developmental abnormalities, usually described as arthrogryposis multiplex congenita (AMC), which includes multiple joint contractures, hypoplasia of the lungs, other malformations and often fetal or neonatal death. The serum of these MG mothers, and others who are asymptomatic, contains antibodies that strongly and very selectively inhibit the ACh-triggered ion channel function of fetal AChR. These inhibitory antibodies persist in the maternal circulation and can transfer a similar condition to the pups after injection into pregnant mice. Interestingly, the first child of mothers of AMC babies is often unaffected and, when present, MG may not be clinically evident in the mother at the time of the first affected birth, suggesting that the maternal immune system may first be sensitized to AChR from the fetus. Moreover, the dominance of fetal AChR-specific antibodies in these women suggested that the B cell response might be clonally restricted.

Here we examined the relationship between inhibitory antibodies and parity in MG patients, and then took advantage of the fact that EOMG patients are treated by thymectomy, providing us with thymic B cells and plasma cells from which to clone and recombine fetal-AChR specific antibodies. We selected combinatorial Fabs from two MG mothers with high levels of fetal AChR-specific antibodies, whose babies had severe AMC. These Fabs proved to be strongly biased towards fetal AChR and were each dominated by one clone with extensive somatic diversification from an already highly mutated consensus sequence. These results suggest that the fetus could be responsible for immunizing the mother during pregnancy, with further diversification occurring subsequently in the thymus.

SUMMARY OF THE INVENTION

In one embodiment, the invention provides an immunotoxin directed at fetal AChR.

In another embodiment of the invention, the immunotoxin directed at fetal AChR further comprises a human Fab fragment based on a human autoantibody or human combinatorial cDNA library.

In another embodiment of the invention, the immunotoxin is a pseudomonas exotoxin A-based single chain Fv IT (35-scFV-ETA).

In another embodiment of the invention, the human Fab fragment of the immunotoxin is derived from a thymus cDNA library of a Myasthenia Gravis patient.

In another embodiment of the invention, there is provided a method of treating a patient with soft tissue tumour comprising the step of exposing the patient to the immunotoxin of the invention.

In another embodiment of the invention, there is provided a method of treating a patient with sarcoma comprising the step of exposing the patient to the immunotoxin of the invention.

In another embodiment of the invention the sarcoma is Rhabdomyosarcoma (RMS).

In another embodiment of the invention there is provided a method of killing cells comprising the step of contacting the cells with the immunotoxin of the invention.

In another embodiment of the invention, there is provided a method of delaying the development of RMS in a patient comprising the step of exposing the patient to the immunotoxin of the invention.

In another embodiment of the invention, there is provided a method of delaying the development of RMS cells, comprising the step of contacting the RMS cells with an immunotoxin of the invention.

In another embodiment, the invention provides a method of inducing immune tolerance in a recipient at risk of developing Myasthenia Gravis comprising the step of exposing the recipient to tolerogenic amount of fetal AChR, thereby inducing immune tolerance in a recipient at risk of developing Myasthenia Gravis.

In another embodiment, the invention provides a method of inducing immune tolerance in a recipient at risk of developing arthrogryposis multiplex congenita comprising the step of exposing the recipient to tolerogenic amount of fetal AChR, thereby inducing immune tolerance in a recipient at risk of developing arthrogryposis multiplex congenita.

In another embodiment, the invention provides a method of preventing Myasthenia Gravis in a recipient at risk of developing Myasthenia Gravis comprising the step of exposing the recipient to tolerogenic amount of fetal AChR, thereby inducing immune tolerance in a recipient at risk of developing Myasthenia Gravis.

In another embodiment, the invention provides a method of preventing arthrogryposis multiplex congenita in newborn of mothers with Myasthenia Gravis comprising the step of administering a tolerating amount of fetal AChR to mothers with Myasthenia Gravis or that are at risk of developing Myasthenia Gravis.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic presentation of the synthesis of scFv fragments based on the recombinant Fab fragments Fab35 and Fab38 using overlap extension PCR.

FIG. 2 demonstrates Colorimetric XTT cytotoxicity assay. Killing of TE671 RMS cells but not control cell lines (IMR32; A431; U937) in vitro by the IT scFv195-ETA that is a derivative of the rat monoclonal antibody mAb195.

FIG. 3 demonstrates FACS analysis and demonstration of specific binding of the IT scFv/Fab35 VL-VH to TE671 RMS cells (red and green curve, fractions from the gradient chromotography).

FIG. 4 demonstrates Colorimetric XTT cytotoxicity assay. Graphic presentation of the viability of TE671 RMS cells (green curve) and control U937 cells (blue curve) at different concentrations of IT scFv/Fab35 VL-VH on day 5 of a representative experiment.

FIG. 5 demonstrates the effect of the immunotoxin treatment on the tumour volume in a mouse transplantation model.

FIG. 6 shows the expression of the AchR γ-subunit in an embryonal RMS before (a) and after (b) chemotherapy. Overexpression and increased percentage of fetal AChR-positive tumour cells after chemotherapy in a three years old boy. Immunoperoxidase (x200).

FIG. 7 shows the inhibition of fetal acetylcholine receptor (AchR) function by sera from AMC-M2 and AMC-M6 from different groups of patients with Myasthenia gravis and healthy controls.

FIG. 8 shows AchR binding by Fab clones. Immunoprecipitation of ¹²⁵I alpha BuTx by representative cloned Fabs from AMC-M2 (a) and from AMC-M6 (b).

FIG. 9 shows the specificity of cloned Fabs. (a) reactivity of AMC-M2 and AMC-M6 serum antibodies and representative Fabs with adults and fetal AchR. (b) competition of AMC-M2 serum and Fab with mABs directed against the indicated AchR subunit. (c) competition of AMC-M6 serum and Fab with mABs directed against the indicated AchR subunit.

FIG. 10 shows evolution of VH and VK clones from AMC-6.

FIG. 11 shows clinical details of the parous mothers.

FIG. 12 shows screening and derivation of anti-AchR Fab clones.

FIG. 13 shows heavy and light chain V genes encoding anti-AchR Fab.

DESCRIPTION OF THE DETAILED EMBODIMENTS OF THE INVENTION

Antibody-based immunotherapy is a well established therapeutic option in high-risk, ErbB2-positive breast cancer, lymphomas or leukemias. In a variety of other tumours, including colon and lung cancer, melanoma, Hodgkin lymphoma and neuroblastoma, immunotoxin-based strategies are being investigated. By contrast, sarcomas, including rhabdomyosarcomas, have not yet been targeted by immunotherapeutics in patients. This has been due to the lack of identified specific sarcoma antigens that are expressed at high levels at the surface of tumour cells but not on relevant normal tissues. In this respect, the description of the fetal AChR isofom as an abundant cell surface antigen of most RMS could be an advance: the invention provides the proof-of-principle that targeting the AChR by an immunotoxin is a feasible approach to kill RMS cells in vitro, while several control cell lines survived the treatment (FIG. 2). The experiments also imply that recombinant ligation of the truncated Pseudomonas exotoxin A (ETA) to the rat 195-scFv fragment and expression in bacteria, did not abrogate the affinity of the anti-AChR scFv moiety for the human AChR. In spite of its good in-vitro efficacy, 195-scFv-ETA is not suitable for use in animal models or patients because it targets the α-subunit that is shared by the adult and the fetal AChR. Therefore, it can be anticipated that 195-scFv-ETA will kill normal muscle fibers in addition to RMS, precluding its clinical usefulness.

Therefore, in one embodiment of the invention, there is provided a construction of an immunotoxin with specificity for the AChR γ-subunit and thus the fetal AChR isoform, the expression of which is close to nil in normal muscle. Two recombinant human Fab fragments that were shown previously to exhibit specific high affinity binding to the γ-subunit of the AChR, were used in the Examples, although the invention is not limited specifically to these Fab fragments. Surprisingly, only the scFv35VL-VH isoform but not the scFv35-VH-VL analogue, and none of the Fab38 derivatives exhibited significant binding to the fetal AChR.

In one embodiment, the invention provides an immunotoxin directed at fetal AChR. In another embodiment of the invention, immunotoxins are molecules that contain targeting domains that direct the molecules to target cells of interest (e.g., effector T lymphocytes, receptors, antigen on cancer cells) and toxic domains that kill the target cells. They are thus useful in pathological conditions such as, cancer, autoimmune diseases, and certain infectious diseases. The toxic domain can be, for example, any of the following toxic polypeptides: ricin, Pseudomonas exotoxin (PE), bryodin, gelonin, α-sarcin, aspergillin, restrictocin, angiogenin, saporin, abrin, pokeweed antiviral protein (PAP), or a functional fragment of any of these toxic polypeptides. The toxic domain can also be diphtheria toxin (DT) or a functional fragment thereof, e.g., a fragment containing amino acid residues 1-389 of DT.

In an embodiment of the invention, a toxic domain of a toxic polypeptide for use as a toxic domain in the fusion proteins of the invention is a fragment of the toxic polypeptide shorter than the full-length, wild-type toxic polypeptide but which has at least 5% (e.g., 10%, 20%, 30%, 40%, 50%, 60%, 70%, 80%, 90%, 95%, 99%, 100%, or even more) of the toxic activity of the full-length, wild-type toxic polypeptide. In vitro and in vivo methods for comparing the relative toxic activity of two or more test compounds are known in the art.

A target cell to which the immunotoxin of the invention binds can be a cancer cell, e.g., a a soft tumour cell, a sarcoma cell, a RMS cell, aneural tissue cancer cell, a melanoma cell, a breast cancer cell, a lung cancer cell, a gastrointestinal cancer cell, an ovarian cancer cell, a testicular cancer cell, a lung cancer cell, a prostate cancer cell, a cervical cancer cell, a bladder cancer cell, a vaginal cancer cell, a liver cancer cell, a renal cancer cell, a bone cancer cell, and a vascular tissue cancer cell.

In another embodiment of the invention, the immunotoxin directed at fetal AChR comprises a human Fab fragment based on a human autoantibody or human combinatorial cDNA library.

In another embodiment of the invention, the immunotoxin (IT) is a pseudomonas exotoxin A-based single chain Fv IT (35-scFV-ETA).

In another embodiment of the invention, the human Fab fragment of the immunotoxin is derived from a thymus cDNA library of a Myasthenia Gravis patient.

In an embodiment of the invention, “antibody fragments” or “Fab fragment” refers to antigen-binding fragments, e.g., Fab, F(ab′)₂, Fv, and single chain Fv fragments.

The Fab fragment may be shorter than the full-length, wild-type targeting polypeptide but which has at least 5% (e.g., 10%, 20%, 30%, 40%, 50%, 60%, 70%, 80%, 90%, 95%, 99%, 100%, or even more) of the ability of the full-length, wild-type targeting polypeptide to bind to its relevant target molecule. The Fab or single chain fragment could also be part of a bi-valent or multi-valent or bi-specific or multi-specific targetting construct. Methods of comparing the relative ability of two or more test compounds to bind to a target molecule are well-known to artisans in the field, e.g., direct or competitive ELISA.

The scFv35VL-VH-ETA IT was functionally effective both in vitro (FIG. 3) and in a mouse model in vivo (FIG. 5). The IT effect on RMS cells in vitro was specific, since control cell lines were not killed. In addition, the IT was not toxic in the 10 mice treated at a dose of 20 μg per day. There were several motifs to use a recombinant human Fab fragment derived from a thymus cDNA library of a Myasthenia gravis (MG) patients as the targeting moiety of the RMS-directed immunotoxin: (a) there were no easily available and well characterized mouse or rat monoclonal antibodies with anti-γ-subnit specificity; (b) human anti-AChR autoantibodies from MG patients are known for their high affinity and specificity that are often maintained in recombinant Fab or single chain Fv derivatives; (c) a human monoclonal autoantibody fragment as the targeting moiety of an immunotoxin is likely to be less immunogenic than a humanized mouse or rat antibody; (d) a human scFv offers the perspective to be applicable in a fully humanized immunotherapeutic agent, be it an immunotoxin (e.g. by using human ribonucleases or linked to other human effectors (e.g. interleukins, chemokines, T-cell receptor fragments, NK-cell receptor fragments, myeloid or dendritic cell signalling molecules).

The latter considerations (i.e. alternative applications of scFv35-derivatives) are likely to be of major practical importance considering both the the cohesive growth of RMS in the clinical setting and the results of our in vivo experiments. As shown in FIG. 5, the “single agent” in-vivo treatment of mice with 35-scFv for 10 days did not cure the animals, since tumour growth was significantly delayed as long as the IT was administered but not totally blocked. In particular, the RMS cells already started to form small but palpable tumours while the IT treatment was still underway (tumour volumes 8+/−7 mm³ as compared to 59+/−9 mm³ in the untreated animals; FIG. 5). This observation suggests that some type of resistance to the treatment had occurred. While our preliminary immunohistochemical studies suggest, that downregulation of fetal AChR surface expression on RMS cells may not be the underlying mechanism, more studies are required to exclude this and other mechanisms that my cause IT resistance. Among the known mechanisms leading to IT resistance, poor vascularisation in sarcomas compared to leukemias or lymphomas almost certainly plays an important role. As to this problem, more prolonged and dose-escalated use of the IT could be one of the options to improve tumour control due to better penetration of the IT to the tumour. Considering the perspective of long-term treatment in humans, the possibility to derive a less toxic and minimally immunogenic, fully humanized IT on the basis of the scFv35 and a human toxin moiety appears desirable.

In addition to dose escalation and extending treatment duration, other possibilities can already been forseen to improve the treatment of RMS with scFv35-based ITs (or other scFv35 derivatives). First, there are now well established molecular biological techniques to stabilize ITs, increase their affinity, and improve their internalization rate into tumour cells. Second, it becomes more and more obvious, e.g. in breast cancer, that single-agent antibody-based therapies as applied here in an experimental setting (FIG. 5) can be made more efficient when combined with “traditional” chemotherapies or targeted application of cytokines, chemokines, antiangiogenic agents or modifiers of degradation pathways that inactivate toxins. Third, as shown in leukemias, the therapeutic effect of ITs can be improved when the expression level of the target antigen and the percentage of target-positive tumour cells is elevated pharmacologically. All startegies appear likely applicable to RMS and scFv35VL-VH-ETA: as shown for a single RMS case previously and for another three new cases here, chemotherapy can drive the differentiation of immature, AChR^(low) RMS cells to more mature rhabdomyoblasts with increased expression of muscle specific proteins, including fetal AChRs (FIG. 7) and Ref. Thus synchronous or metachronous application of chemotherapy and a fetal AChR IT appears reasonable for testing once the IT has been optimized.

In another embodiment of the invention, there is provided a method of treating a patient with sarcoma comprising the step of exposing the patient to the immunotoxin of the invention. The step of exposing may be either by oral, topical or by injection.

In another embodiment of the invention the sarcoma is Rhabdomyosarcoma (RMS).

In another embodiment of the invention there is provided a method of killing cells comprising the step of contacting the cells with the immunotoxin of the invention.

The step of contacting is by either direct administration to the cells or to the media which surround the cells.

In another embodiment of the invention, there is provided a method of delaying the development of RMS in a patient comprising the step of exposing the patient to the immunotoxin of the invention.

In another embodiment of the invention, there is provided a method of delaying the development of RMS cells, comprising the step of contacting the RMS cells with an immunotoxin of the invention. As can be seen in the examples section, a recombinant ETA-based immunotoxin derived from one of the human autoantibody fragments killed human RMS cells in vitro and resulted in a significant delay of tumour development.

In another embodiment of the invention, the method of treating and/or of killing a cell, and/or of delaying the development of a tumour further comprising a step of contacting or exposing the cells to a chemotherapeutic agent.

The chemotherapeutic agent is selected from the group consisting of a DNA-interactive agent, alkylating agent, antimetabolite, tubulin-interactive agent, hormonal agent, Asparaginase and hydroxyurea or without limitation from the group consisting of Asparaginase, hydroxyurea, Cisplatin, Cyclophosphamide, Altretamine, Bleomycin, Dactinomycin, Doxorubicin, Etoposide, Teniposide, paclitaxel, cytoxan, 2-methoxycarbonylaminobenzimidazole carbamate and Plicamycin

In summary, the invention shows that the fetal AChR is the so far most specific and potentially useful target antigen of human RMS to be recognized by an immunotoxin. The IT (Immunotoxin) described here (scFv35VL-VH-ETA) was effective both in vitro and in vivo against RMS cell lines. It is the first sarcoma directed agent based on a fully human autoantibody fragment. ScFv35VL-VH-ETA is a promising candidate for further preclinical testing, including its molecular modification towards better stability in vivo, higher binding affinity and coupling of human toxins or other effectors like cytokines or chemokines.

Rhabdomyosarcoma (RMS) or other tumour with rhabdomymatous differentiation is the leading malignant soft tissue tumour in children with high mortality rates in spite of modern multimodality treatments. Since strong expression of the γ-subunit of the acetylcholine receptor (AChR), defining the human fetal-type AChR isoform, is virtually specific for rhabdomyoblasts, RMS cells could be killed by immunotoxin (IT) directed at the fetal AChR. Two fully human Fab-fragments with specificity for fetal AChR, obtained from a combinatorial library created from a cDNA library derived from the thymus of a myasthenia gravis patient, and generated a pseudomonas exotoxin A-based single chain Fv IT (35-scFV-ETA). 35-scFV-ETA killed human embryonal and alveolar RMS cell lines in vitro and delayed RMS development after transplantation into mice. 35-scFV-ETA was specific to fetal AChR since it did not kill AChR-negative cell lines or HEK cells transfected with RNA for human adult-type AChRs indicating that it should not affect AChRs at the normal muscle endplates. 35-scFv-ETA is the first cancer-directed immunotoxin with a fully human antibody moiety and the first IT targeting an antigen that is virtually specific for RMS. 35-scFv-ETA appears promising for further preclinical evaluation as a therapeutic tool against high-risk RMS.

The invention shows, for the first time, that antibodies inhibiting the ion channel function of fetal AChR are common not only in mothers of AMC babies (Riemersma et al 1997), but also in women who developed MG after pregnancy. By contrast, such antibodies were uncommon in women who presented before pregnancy. In addition, the many high affinity Fabs that was cloned from the thymus of two AMC mothers showed a very strong preference for fetal AChR and a biased usage of VH3 genes. The striking diversification of the Fabs in each woman from a common, but already highly mutated, germ-line sequence, shows that the autoantibodies can arise via successive rounds of antigen-driven selection in a few clones. These results suggest a scenario in which oligoclonal responses to fetal AChR, including some that are fetal AChR inhibitory, are the initiating event in women who develop MG after pregnancy, irrespective of whether the extent of fetal AChR inhibitory antibodies is sufficient to cause AMC. Immunization by the fetus is, therefore, another contributory factor to the female bias evident in many autoimmune diseases, in addition to the established hormonal influences and the possible roles of fetal-maternal microchimerism.

The Fabs that were cloned from the two unrelated AMC-mothers show remarkable similarities. They clearly have high affinities, since they immunoprecipitate AChR at ˜500 pM and, despite being monomeric, they also efficiently block binding of bivalent mAbs. In theory, the predominance of fetal AChR in the ischemic muscle extracts that were used might have created a bias in screening towards fetal AChR-binding Fabs. However, the MIR on the a subunits is thought to be the target of the majority of AChR antibodies in MG; although some may have a preference for the fetal isoform, it was possible to detect them with adult AChR, and to inhibit them with the anti-MIR mAb.

The efficient screening and cloning of Fabs binding specifically to fetal AChR implies a striking dominance of this specificity in the AMC mothers' thymic cells. Furthermore, it was shown a single dominant family of clonally related Vκ sequences in AMC-M6 and of VH3 sequences in both patients. In AMC-M2, the same clonally related VH3-07 sequences predominated whether they were isolated together with κ or λ light partners and irrespective of the screening procedure used (Table 2). A parallel restriction in Fabs recognizing a minority epitope, contrasting with heterogeneity of those against a dominant region, has recently been reported for thyroid peroxidase in a patient with Graves' disease. The VH3 germline genes of our two clonal families are both commonly used by normal blood B cells, as is the Vκ1 02/12 that predominated in AMC-M6 and recurred in AMC-M2. A PCR bias seems very unlikely since the five non-specific Fabs isolated from our AMC-M2 VH/Vκ library did not include the VH3-07, VH3-21 or Vκ1 02/12 genes, and VH3 genes are not over-represented in Fabs cloned, using the same primers, from autoimmune thyroid tissue.

Since the dominant VH3-07 used by AMC-M2 was paired with so many unrelated light chains (both κ and λ), its VH probably makes the major contribution to binding specificity, as has frequently been observed with other antibodies. With AMC-M6, by contrast, the recurring usage of highly mutated and clonally related light chains in exclusive combination with the dominant VH3-21 suggests that they also have a major influence on specificity. There are long-established precedents for that in certain heritable restricted responses in mice to well defined epilopes, and also in human autoantibodies.

Importantly, the deduced ancestral sequence for each of the dominant families is already highly mutated (with 32, 44 and 25 shared differences from germline for AMC-M2 and AMC-M6 VH, and AMC-M6 Vκ respectively) indicating that the progenitor B cells had already undergone antigen-selection in GC before further refinement in the thymus. Almost all of the present sequences show multiple mutations, especially in the CDRs where replacement: silent ratios were often high. Of particular interest is the evidence of convergent mutations in the two patients' VH and especially in the recurrent ²²SRASET²⁸ sequences in Vκ, suggesting that these Fabs may be recognizing a dominant fetal AChR epitope. Together with the abundant mutations, the high R:S ratios in the CDRs, and the branching patterns of their evolution, these results strongly suggest that each Fab is the end-product of successive stages of antigen-driven clonal proliferation in GC, as also found after Haemophilus vaccination in subjects with preexisting B cell memory. One can envisage that, over the preceding years, and four and two pregnancies respectively, the memory B cells that were initially generated could re-activate and/or re-enter GC for further rounds of mutational refinement. The striking patterns of oligoclonal evolution/diversification that was have observed in the fetal AChR-specific Fabs from both AMC mothers, are very reminiscent of those noted in a mouse model with spontaneous SLE, where a surprising variety of specificities stemmed from remarkably few ancestral B cells.

On the basis of these results, it is suggested that the initial immunization is by the fetus, generating a restricted group of mutated ‘progenitor’ memory cells, possibly in lymph nodes draining the uterus. Consequently, circulating antibodies are produced against fetal AChR; they attack rare myoid cells in the thymus that express this isoform, releasing antibody:AChR complexes that provoke local GC formation very efficiently. The progenitor memory cells are attracted by the complexes trapped in these GC, and undergo further rounds of mutation and selection, culminating in the expanded populations of clonally related B cells and plasma cells that we have detected (see Table 3,which is in FIG. 13 and FIG. 4). Several autoimmune diseases show both a strong female bias and onset in early adulthood (e.g SLE, thyroid disease, multiple sclerosis and EOMG); there is evidence for the importance of hormonal influences in the autoimmune response, as well as potential X chromosome contributions in these diseases. Persisting fetal—maternal microchimerism has also been invoked because of the recent findings of fetal or maternal RNA in scleroderma lesions. There are already well known examples of alloimmunization of mothers, for example, by fetal erythrocytes or platelets. While it might be informative to look for allotypic variations in the AChR γ subunit, the AChR is remarkably autoimmunogenic, even without adjuvant. Although the majority of EOMG female patients present before pregnancy, our results suggest that autoimmunization by the fetus is another possible route to maternal MG, perhaps enhanced by presentation by semi-allogeneic (fetal) cells.

In another embodiment, the invention provides a method of inducing immune tolerance to fetal AchR in a recipient at risk of developing Myasthenia Gravis comprising the step of exposing the recipient to tolerogenic amount of fetal AChR, thereby inducing immune tolerance in a recipient at risk of developing Myasthenia Gravis.

In another embodiment, the invention provides a method of inducing immune tolerance to fetal AchR in a recipient at risk of developing arthrogryposis multiplex congenita comprising the step of exposing the recipient to tolerogenic amount of fetal AChR, thereby inducing immune tolerance in a recipient at risk of developing arthrogryposis multiplex congenita.

The recipient at risk of developing arthrogryposis multiplex congenita is a newborn to a pregnant mother, who has sympthoms of Myasthenia Gravis, or who has a history family of either Myasthenia Gravis or arthrogryposis multiplex congenita.

The recipient at risk of developing Myasthenia Gravis is a mother who has a history family of Myasthenia Gravis and in another embodiment, who has a history family of Myasthenia Gravis in pregnancy.

In another embodiment, the invention provides a method of preventing Myasthenia Gravis in a recipient at risk of developing Myasthenia Gravis comprising the step of exposing the recipient to tolerogenic amount of fetal AChR, thereby inducing immune tolerance in a recipient at risk of developing Myasthenia Gravis.

In another embodiment, the invention provides a method of preventing arthrogryposis multiplex congenita in newborn of mothers with Myasthenia Gravis comprising the step of administering a tolerating amount of fetal AChR to mothers with Myasthenia Gravis or that are at risk of developing Myasthenia Gravis.

In one embodiment of the invention, inducing tolerance may be referred to unresponsiveness to an antigen without inducing a prolonged generalized immune deficiency. Tolerance represents an induced depression in the response to an antigen.

As used herein, “antigen” refers to a substance, which elicits an immune response. The antigens of the invention to which tolerance is induced may or may not be exogenously derived relative to the host. For example, the method of the invention may be used to induce tolerance to an “autoantigen”. An autoantigen is a normal constituent of the body that reacts with an autoantibody.

Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains. In case of conflict, the present document, including definitions, will control. Preferred methods and materials are described below, although methods and materials similar or equivalent to those described herein can be used in the practice or testing of the present invention. All publications, patent applications, patents and other references mentioned herein are incorporated by reference in their entirety. The materials, methods, and examples disclosed herein are illustrative only and not intended to be limiting.

EXAMPLES Experimental Procedures Generation of Anti-AChR Fab and scFv Fragments

The cloning and expression of the high-affinity rat 195-scFv fragment derived from the anti-AChR rat antibody mAb195 with specificity to the α67-76 sequence (main immunogenic region, MIR) has been described previously. The generation of the recombinant human Fab fragments Fab35 and Fab38 using a combinatorial cDNA thymus library of a patient with thymic lymphoid hyperplasia and production autoantibodies specific for the anti-AChR γ-subunit has been described as well. ScFv fragments were produced from Fab35 and Fab38 by overlap extension as shown in FIG. 1. For each Fab, two recombinant scFv fragments were generated: one in a VH-VL, the other in a VL-VH orientation. The respectve primers were as follows:

Primers for scFv from Fab35/38 in Orientation V_(H)-V_(L)

scFv/Fab35 V_(H)5′ SfiI: 5′-AGTCTAAGGTTCGGCCCAGCCGGCCTCGGGG (SEQ ID. No. 1) GGCGACTTGGTCCAGCCGGGGGGG-3′

scFv/Fab38 V_(H)5′ Sfi I: 5′-AGTCTAACGTTCGGCCCAGCCGGCCTCGGGGG (SEQ ID. No. 2) GAGGCGTGGTCCAGCCGGGGGGG-3′

scFv/Fab35/38 V_(H)3′ Linker: 5′-GCCACCCGACCCACCACCGCCCGAGCCACCGC (SEQ ID. No. 3) CACCTGGAGAGACGGTGACCGTTGTCCCTTGGC C-3′

scFv/Fab35/38 V_(L)5′ Linker: 5′-GGCTCGGGCGGTGGTGGGTCGGGTGGCGGCGG (SEQ ID. No. 4) ATCAGTGATGACCCAGTCTCCA-3′

scFv/Fab35 V_(L)3′ Not I: 5′-TGCTGGTGCGGCCGCTTTGATCTCCAGCTTGG (SEQ ID. No. 5) TCCC-3′

scFv/Fab38 V_(L)3′ Not I: 5′-TGCTGCTGCGGCCGCCGTGATCTCCAGCTTGG (SEQ ID. No. 6) TCCC-3′ Primers for scFv from Fab35/38 in Orientation V_(L)-V_(H)

scFv/Fab35/38 V_(L)5′ Sfi I: 5′-ATGGCTCAGGGTTCGGCCCAGCCGGCCGTGAT (SEQ ID No. 7) GACCCAGTCTCCA-3′

scFv/Fab35 V_(L)3′ Linker: 5′-GCCACCCGACCCACCACCGCCCGAGCCACCGC (SEQ ID. No. 8) CACCTTTGATCTCCAGCTTGGTCCC-3′

scFv/Fab38 V_(L)3′ Linker: 5′-GCCACCCGACCCACCACCGCCCGAGCCACCGC (SEQ ID. No. 9) CACCCGTGATCTCCAGCTTGGTCCC-3′

scFv/Fab35 V_(H)5′ Linker: 5′-GGCTCGGGCGGTGGTGGGTCGGGTGGCGGCGG (SEQ ID. No. 10) ATCATCGGGGGGCGACTTGGTCCAGCCGGGGG GG-3′

scFv/Fab38 V_(H)5′ Linker: 5′-GGCTCGGGCGGTGGTGGGTCGGGTGGCGGCG (SEQ ID. No. 11) GATCATCGGGGGGAGGCGTGGTCCAGCCGG GGGGG-3′

scFv/Fab35/38 V_(H)3′ Not I: 5′-TGCTGCTGCGGCCGCTGGAGAGACGGTGACG (SEQ ID. No. 12) GTTGTCCCTTGGCC-3′

Production and Purification of the Immuntoxines (IT)

The protocol for the production and purification of recombinant ITs was published recently. In brief E. coli strain BL21DE3 lys (Stratagene, Heidelberg, Germany) were transformed with the corresponding plasmids. A preculture of 10 ml LB medium/50 μg/μl Kanamycin was inoculated and incubated for 6-8 hours at 37° C. 5 ml of this preculture were added to 500 ml Terrific Broth/0,5 mM ZnCl/50 μg/μl Kanamycin and shaken overnight at 26° C. After 14-16 hours of incubation, the 500 ml culture was divided on 5 1 Terrific Broth/0.5 mM ZnCl/50 μg/μl Kanamycin (in 20 1 l-flasks each containing 250 ml medium) and incubated for further 3-5 hours at 26° C. After adding Sorbitol, NaCl and Betain to a final concentration of 0.5M, 4% and 110 mM respectively, cultures were grown for 30 min. and protein production was induced by IPTG (final concentration 1-2 mM/l) and overnight shaking.

On the next day bacteria were harvested by centrifugation (3700 g, 4° C.), pellets were washed in 1× wash buffer (75 mM Tris pH8.8, 4% NaCl and 10% glycerol) resuspended again in 1× wash buffer and frozen in liquid nitrogen. Bacteria were lysed in 1× lysis buffer (75mM Tris/HCl pH 8.8, 300 mM NaCl, 10% glycerol, 5 mM DTT, PMSF/EDTA, 5 mM EDTA), sonified and centrifugated (25000 g, 4° C., 60 min.). The supernatant was filtered, desalted by FPLC and desalting column (Amersham Biosciences, Heidelberg, Germany) (flow rate 6 ml/min). The pooled protein fractions were purified and eluted by a Nickel-NTA-Agarose column (Quiagen, Hilden, Germany) with 1× elution buffer (75 mM Tris/HCl pH 8.8, 10% glycerol, 1 M NaCl, 0.5 M Imidazol) and a flow rate of 6 ml/min.

Subsequently aliquots of the corresponding fractions were loaded on a 6% SDS acrylamidgel and prtoein was blotted onto nitrocellulose membrane as described recently.

Colorimetric XTT Cytotoxicity Assay and RMS Cell Lines

The assay is based on the metabolization of the yellow tetrazolium salt XTT (conzentration: 1.49 mM, Roche, Molecular Biochemicals, Mannheim, Germany) to the orange and water soluble chromogen formazan. The concentration of formazan is measured at 450 nm und 650 nm by a 96 well plate elisa detector (MWG Biotech, Ebersberg, Germany). Supernatants were checked for chromogen development 96 h after the beginning of IT application to cell cultures grown at subconfluent density.

As IT targets and controls, the following cell lines were tested: Embryonal RMS cells (TE761), alveolar RMS (RD); muscle-type AChR-negative controls: IMR32 (neuroblastoma); U937 (hematologic precursor cells line); A431 (squamous cell carcinoma cell line).

Generation and Treatment of RMS in an In-Vivo Nude Mouse Model

Subcutaneous injection of 5×10⁶ TE671 cells suspended in DMEM serum-free culture medium on day 1 served to generation RMS in nude mice (n=10). Starting on day 2 up to day 10, mice received either 10 mg scFv3 5-ETA (n=6) or saline (n=4) twice daily intraperioneally as described for othet toxins previously. Tumour size was checked manually on day 5 and then every third day up to day 20, when mice were killed and autopsied for detection of metastases. Tumour explants were measured for size and subdivided for a) shock freezing; b) embedding in paraffin; c) FACS analysis using scFv35-ETA (and a polyclonal antiserum, Santa Cruz).

Immunhistochemistry and Detection of AChR α and γ Subunit mRNA by Semiquantitative Multiplex PCR

Immunhistochemical detection of the fetal AChR on frozen and paraffin-embedded sections was achieved by a polyclonal goat antiserum specific for the AchR γ-subunit (Santa Cruz, Santa Cruz, USA) as described previously.

For simultaneous amplification of the α and γ subunit mRNA of the AchR and the to determination of the γ/α ratio, a semiqunatitative multiplex RT-PCR was performed as reported previously.

Clinical Material

Patient material was obtained with informed consent and Ethical Committee approval. Sera from MG patients were collected in Professor John Newsom-Davis' clinic, before thymectomy or immunosuppression, and stored at −20° C. Antibodies to AChR were measured as previously described (Vincent et al 1995; Riemersma et al 1997). AMC-M2 and AMC-M6, described in (Riemersma et al 1997; Jacobson et al 1999), had had four and two babies, respectively, with severe AMC (fatal in all but one), but the diagnosis of maternal MG was made only after the birth of the AMC babies. After therapeutic thymectomy and immunosuppressive therapies for their MG, and plasma exchange during pregnancy, both mothers have subsequently had successful pregnancies with minimally affected babies (unpublished observations). Thymus cell suspensions were prepared and cryopreserved as in (Willcox, Newsom-Davis and Calder 1983). From both patients, these cells spontaneously produced high levels of anti-AChR antibodies in culture that were reduced by pokeweed mitogen, typical of plasma cell behavior (Willcox, Newsom-Davis and Calder 1983); these antibodies showed a strong preference for fetal AChR, and also inhibited its ion channel function (not shown), as did serum antibodies from AMC-M2 (Riemersma et al 1997) and AMC-M6.

Measurement of Inhibition of AChR Function in TE671 Cell Line

AChR function was measured in TE671 cells that express only fetal AChR, as previously described in (Riemersma et al 1997). Carbachol-induced ²²Na⁺ flux was measured over 1 minute, and the internalized ²²Na⁺ was counted on a Packard Autogamma counter. The inhibitory effect of the sera was tested by incubating the cells with 25 μl of serum in 500 μl Hepes Locke buffer (ie. 1:20) for 30 minutes. The results are expressed as % inhibition, with 0% being the cpm in Hepes Locke buffer alone and 100% inhibition being the cpm in cells tested in the presence 1 μg/ml of the antagonist α-bungarotoxin (α-BuTx). Those sera producing more than 50% inhibition were then retested at higher dilutions and the results are presented as % inhibition/μl of serum.

Combinatorial Ig Gene Library Construction

The Fab library was made from cDNA after reverse-transcription from mRNA obtained from thymic cells (Farrar et al 1997). We used an anti-sense primer for an IgG1 CH1 sequence (almost identical to its IgG3 homolog (Kabat et al 1991)) to amplify VH chain cDNA by PCR in combination with a panel of sense oligonucleotide primers designed to include VH1-VH6 gene families: further panels amplified the Vκ or Vλ gene families, as described in (Farrar et al 1997). The PCR products were ligated into Immunozap H or L bacteriophage vectors (Stratagene). Subsequently, the heavy and light chain DNAs were ligated into Immunozap to yield combinatorial libraries, as detailed in (Farrar et al 1997) and (Chazenbaik et al 1993).

Screening the Library for AChR-Binding Fabs

Positive clones were identified essentially as in (Farrar et al 1997). Muscle extracts from denervated muscles were labeled with 2 nM ¹²⁵I-α-BuTx (Amersham International, Amersham, UK; specific activity 2000 Ci/mmol) and used to screen the unamplified combinatorial library in XL1-Blue cells by filter-lift assays. Positive plaques were identified by autoradiography, cloned to homogeneity, and the heavy and light chain genes were sequenced in both directions by the dideoxy chain termination method (Sanger et al 1977). To obtain soluble Fabs, the XL1-Blue cells were induced with 1 mM isopropyl-thio-galacto-pyranoside (Sigma, St Louis, Mo.) overnight. The cells were then pelleted and freeze/thawed in 10 mM Tris buffer pH 8.0 containing protease inhibitors (see (Farrar et al 1997)). The suspension was sonicated and cleared by centrifugation, to leave a Fab-containing lysate.

Characterizing Fab Reactivity with AChR

The lysates were used without further purification. They were incubated with AChR (either from muscle extracts, from TE671 cells or from a transfected subline that expresses predominantly adult AChR (Beeson et al 1996)), labeled with ¹²⁵I-α-BuTx. After 2 hours at 20° C., carrier normal human serum was added plus a goat anti-human IgG (Lawrance Laboratories, Western Australia) which precipitated the Fab-AChR complexes efficiently. Competition with mAbs (Jacobson et al 1999) was measured by preincubating the ¹²⁵I-α-BuTx-AChR with 50 μl of each Fab overnight at 4° C. and then adding excess (0.1 μl of ascites) of each mAb (see (Farrar et al 1997)). Sheep antibody to mouse IgG (which did not precipitate human Fabs) was then added to precipitate mAb-AChR complexes. All results were compared with precipitation in the presence of a control Fab that did not bind AChR, and expressed as the % inhibition by each Fab of the binding of the indicated mAb (see also (Whiting et al 1986)).

Bioinformatics

Sequences were compared with the human VBASE directory of immunoglobulin genes (Tomlinson et al 1997) using DNAPLOT (Müller, W. Institut für Genetik, Köln) to determine the best matching germline V-gene segments. Sequences with the same gene rearrangement and common CDR3s were judged to be clonally related. The numbers of somatic mutations over the VH and Vκ regions were determined, and ratios of replacement to silent mutations (R:S ratio) were calculated for framework (FWR) and complementarity-determining regions (CDR). Amino acid numbering and FWR and CDR positions were previously defined by Kabat et al (1991). Genealogical trees were constructed for sets of related genes by analysis of shared and unshared mutations using phylogenetic analysis using parsimony (PAUP) (Swofford 1993). Independent genes were also compared for the occurrence of convergent mutations.

EXPERIMENTAL RESULTS Example 1 An IT Targeting the Main Immunogenic Region (MIR) of the AChR α-Subunit Kills RMS Cells In Vitro

In a first step we generated an ETA-containing IT based on a recombinant single chain Fv fragment that had been derived from the well characterized rat monoclonal antibody mAb195 against the main immunogenic region (MIR) of the human AChR α-subunit. This model IT (scFv-195-ETA) specifically bound to several RMS cell lines as revealed by FACS (not shown). Binding of scFv-195-ETA to TE671 RMS cells could be competed using unlabeled scFv-195 (not shown). In addition, scFv-195-ETA killed the RMS cell lines TE671, FLOH-1 and RD in a dose-dependant manner, while non-RMS epithelial, hematopoietic and neurogenic cell lines (A431; U937; IMR32, respectively) without muscular AChR expression were not killed (FIG. 2). These experiments were considered as “proof-of-principle” that specific targeting of the AChR by an IT can kill AChR-positive RMS cells. Since the α-subunit is shared by RMS cells and normal skeletal muscle, scFv195-ETA is not applicable in a therapeutic setting in humans.

Example 2 Synthesis of an IT Targeting the γ-Subunit of the Human Fetal AChR In Vitro

Since the fetal AChR, specified by the γ-subunit, is virtually absent in non-neoplastic innervated skeletal muscle but expressed in most RMS, we generated an ETA-based IT with specificity for the AChR γ-subunit. Synthesis started from two recombinant human Fab fragments (Fab35 and Fab38) that were derived from a combinatorial cDNA library derived from the thymus of a seropositive MG patient with high titers of anti-AChR γ-subunit antibodies. Although cloning of the two Fab fragment cDNAa into the ETA-vector resulted in constructs that were in frame as revealed by sequencing, they were inefficiently translated in transformed E. coli BL21DE3, resulting in several abnormal protein fragments of 70-80 kd instead of the expected 120 kd full-lenghth IT (not shown). Therefore, we used the Fab35 and Fab38 cDNAs to produced single-chain Fv (scFv) fragments by overlap extension PCR (FIG. 1). VH and VL fragments were linked by a 36 nucleotide (Glycin/Serine) spacer. ScFV fragments with a VH-VL and a VL-VH orientation were produced from both the Fab35 and Fab38, yielding 4 differentent scFvs.

These 4 scFv cDNAs were cloned into the ETA vector pBM-1.0, checked for proper ligation by sequencing and expressed in E. coli BL21DE3. The 4 expressed scFV-ETA proteins were purified and analysed by FACS analysis for binding capacity, using TE671 rhabdomyosarcoma (RMS) cells as targets (s. Materials and Methods). Among the 4 different ITs expressed, only the Fab35 derivative 35scFv/VL-VH-ETA exhibited significant binding to TE671 cells (FIG. 3).

The 35scFvV_(L)-V_(H)-ETA was further analysed for killing activity in an in vitro cytotoxicity assay using TE671 cells as targets. As shown in FIG. 4, the IT killed TE671 cells but not several AChR-negative control cell lines (IMR32, HL60, U937) in a dose-dependant manner. Killing activity of the IT could be prevented by blocking the IT binding site using “cold” (ETA-deficient) 35scFvVL-VH. In further assays, specific killing of other RMS cell lines (FLOH-1; RD;) was demonstrated.

Example 3 An AChR γ-Subunit-Targeting Immunotoxin Delays Tumour Development in a Mouse RMS Transplantation Model

35 scFvVL-VH-ETA was further investigated for its in vivo killing capacity using RD RMS cells as targets in a RMS transplantation mouse model. Toxicity tests revealed that intravenous injection of 10 μg IT per mouse had no apparent advers effect on the mice tested (n=10) in terms of mobility, weight development, and survival (up to 60 days). Therefore, this dose of IT was co-injected with RD cells (10⁷ per mouse) (n=10) intravenously. Saline injections served as negative controls (n=4). Twice daily injections of 10 μg IT per mouse were repeated up to day 10. Tumour development was monitored by daily inspection and palpation. As shown in FIG. 5, 100% of mock injected mice exhibited palpable tumours day 7 of the experiment, and clearely visible subcutaneous tumours by day 10 (volume: 59+/−9mm³). By contrast, no or minimal tumours were detectable in IT-injected mice by day 4 and the tumour remained generally invisible (but palpable in all cases) by day 10 (volume: 8+/−7 mm³). Up to day 12 the tumours in the treated group were significantly smaller than in the controls (FIG. 5). After finishing the daily IT injections, however, subcutaneous tumours began to enlarge rapidly in the test group as well and were visible by day 14 in all animals, when the size of the tumours in test group was no longer signicantly different from the untreated controls (FIG. 5).

Example 4 IT Effects on Tumour Morphology and AChR Levels In-Vivo (to be Completed)

Mice were killed 20 days after the first injection. By histology, viable tumour was documented in all cases. Tumour volumes at that time were not significantly different in the treated (920+/−660 mm³) and untreated control group (1544+/−342 mm³) (p>0.10). By FACS analysis, no significant difference between the tumour cells of treated animals as compared to the control group could be seen (not shown). This finding was supported by immunohistochemistry, with tumours in the test and control group exhibiting expression of AChR γ-subunit at comparable levels and in a similar percentage of tumour cells. In agreement with these morphological finding RT-PCR revealed similar levels of AChR α- and γ-subunit mRNA expression in both groups of mice.

Example 5 Detection of Fetal AChR in Human RMS Biopsies Following Chemotherapy

Since immunotoxin treatments in general can be improved by increasing expression levels of the target antigen on as many tumour cells as possible, we wondered whether chemotherapy might increase expression of fetal AChRs on RMS cells escaping current chemotherapies. As shown for a prototypic case in FIG. 6, vital RMS cells detected inside a second-look resection specimens (n=3) expressed high levels of fetal AChR on virtually 100% of tumour cells (FIG. 6 a). By contrast, the same tumour prior to chemotherapy exhibited both less intense AChR expression levels and a significantly lower number of AChR(+) tumour cells (FIG. 6 b).

Example 6 Antibodies Inhibiting Fetal AChR Function in MG

The records were searched for women with MG who had had children before their first available serum sample. We found twelve women with generalized MG who had had 1-3 children each by the time of sampling, which was before thymectomy or immunosuppressive treatment (Parous MG; see Table 1 in FIG. 11); in all but one case, MG presented during or after pregnancy. We compared the results with those of 12 women who had not had children at the time of sampling (Non-parous MG), 11 male MG patients and 12 healthy controls. Two AMC-M sera were used as positive controls. As expected, the age at MG onset was higher in the Parous MG than the Non-parous MG women (mean±SD; 30.7±7.8 compared with 19.9±5.3), but there was no substantial difference in the total levels of anti-AChR antibodies on routine testing (22.26±10.6 nM compared with 26.05±12.7 nM). Male MG patients were of a similar age to parous females (30.9±8.4) and with similar anti-AChR values (18.5±12.7). To assay the levels of fetal-AChR inhibitory antibodies, we measured the effects of the sera on agonist-induced ²²Na flux into TE-671 cells. Healthy sera did not inhibit flux appreciably (0.02±0.58% compared to results in Hepes-Locke buffer alone). Inhibition of flux by the Parous MG sera was greater than that by the Non-parous (p=0.023; Mann Whitney one-tailed; FIG. 1) or male MG sera. These results suggest that pregnancy can influence the specificity of AChR antibodies in MG patients, and may initiate the response in some susceptible individuals.

Example 7 Cloning Anti-AChR Fabs from Thymic Combinatorial Libraries

In order to examine the clonal origins of fetal-specific AChR antibodies, we characterized Fabs cloned from unamplified VH/Vκ cDNA libraries prepared from thymic cells of AMC-M2 and AMC-M6, after screening 2.0-2.5×10⁵ clones. It was relatively easy to detect AChR-specific plaques by blotting the expressed Fabs with ¹²⁵I-αBuTx-AChR solubilized from human muscle, and to clone the positives by further rounds of screening (FIG. 8 a); the results are summarized in Table 2. Fetal AChR inhibitory antibodies compete with one of the two α-BuTx-binding sites (Riemersma et al 1997), leaving the second site available for detection with ¹²⁵I-α-BuTx. Therefore, in order to clone Fabs that might compete with α-BuTx for binding to the fetal AChR, we re-screened the AMC-M2 library with unlabeled muscle extract, allowing the AChR to bind before we applied the ¹²⁵I-α-BuTx. A further 25 clones were thus isolated and two characterized in detail. We also prepared a parallel VH/Vλlibrary from AMC-M2 and isolated another 25 clones (Table 2 in FIG. 12).

Example 8 Specificity of AMC-M Fabs for Fetal AChR

Both AMC-M2 and AMC-M6 libraries yielded Fabs that efficiently immunoprecipitated ¹²⁵I-α-BuTx-human AChR, in most cases precipitating all of the available receptor. Examples of titrations of the Fabs are shown in FIGS. 8 b and c; the efficient binding by monomeric Fabs of soluble AChR (at ˜500 pM final concentration) indicates a high affinity. The muscle extracts used for screening and precipitations were from amputees with ischemic disease; although these have a preponderance of fetal over adult AChR (Vincent and Newsom-Davis 1984), antibodies to the α, β or δ subunits bind similarly to both isoforms (Tzartos et al 1998; Fostieri, Beeson and Tzartos 2000). We therefore tested the Fabs against AChR extracted from TE671 cells that express only fetal AChR, or from a TE671 subline that expresses predominantly (>90%) adult AChR (Beeson et al 1996). The two AMC-M sera preferred the fetal isoform, although they also showed some reactivity with adult AChR (FIG. 9 a). However, all of the Fabs, with one exception (Fab 8H/K from AMC-M6), bound almost exclusively to fetal AChR. Even the low apparent reactivity with the adult AChR preparation could be due to the 10% contamination with fetal AChR (Beeson et al 1996).

To confirm this striking finding, we also tested the Fabs' ability to block the binding of mAbs specific for human fetal AChR γ subunits. Both AMC-M2 and AMC-M6 serum antibodies blocked binding of each of the two mAbs specific for fetal AChR (FIGS. 3 b and c), as did all the Fabs except 8K from AMC-M6, which instead blocked binding of the mAb with β subunit-specificity (FIG. 3 c). These experiments confirm the high affinity for fetal AChR of the Fabs and most of the serum antibodies, and their probable γ subunit-specificity.

It was also important to test the Fabs for inhibition of fetal AChR ion channel function. Disappointingly, only Fab 35K from AMC-M2 showed appreciable activity and that only at relatively high Fab concentrations (50% inhibition at 100 μl of Fab). There was no effect on adult AChR (data not shown). In parallel experiments, cross-linking the cloned Fabs with secondary antibodies did not increase the degree of inhibition. Indeed, inhibition of function did not require divalent antibodies, since it was readily measurable with monovalent Fabs prepared from both donors' serum Ig (data not shown).

Example 9 Sequences of the AMC-M2 Fab VH and Vκ Genes

The V-genes were sequenced and compared with those in the human Ig VBASE directory (Tomlinson et al 1997) to identify both the closest germline sequence and the number of somatic mutations (Table 3 in FIG. 13). All the AMC-M2 Fabs were specific for fetal AChR and used the same VH3-07/D/JH6b combination, regardless both of the screening procedure used to identify them (Table 2 in FIG. 12) and of their exact κ (Table 3 in FIG. 13) or λ light chain partner (not shown). They had the same CDR3 length and the majority shared 32 ‘consensus’ VH mutations, demonstrating a common clonal origin from a previously-mutated progenitor. Replacement:silent (R:S) ratios were higher in the first two hypervariable (CDR) loops than in the framework regions (FWR), strongly suggesting antigen-driven selection for antibody specificity/affinity.

By contrast, the AMC-M2 kappa light sequences were heterogeneous; despite their similar binding specificity, these Fabs used a variety of Vκ (Table 3 in FIG. 13), which varied substantially both in numbers of mutations and R:S ratios. Although two Fabs used the Vκ 02/12 gene, their different Jκ usage demonstrates that they are independent gene rearrangements. This contrasting VH restriction and Vκ diversity suggests that the heavy chain was primarily responsible for the fetal AChR-binding specificity.

Example 10 Recurring VH and Vκ Usage by AMC-M6 Fabs

Table 3 also summarizes the genes encoding the second donor's anti-AChR Fabs. Fab 8H/K is the first human AChR β subunit-specific autoantibody to be cloned. It uses the relatively uncommon VH4-61 gene in combination with the commonly expressed VK1 02/12 Vκ gene. Both the heavy and light chains are highly mutated, with higher R:S ratios in the CDR than the FWR regions (Table 3 in FIG. 13). One of the fetal AChR-specific Fabs (Fab 1H/K) used a highly mutated VH3-23/DH3.3/JH6b plus a Vκ4 B3/Jκ 3 gene (Table 3 in FIG. 13). All of the others used the same pair of VH3-21/D/JH5b heavy and Vκ 02/12 light chain genes, again with Jκ4 (Table 3); the frequency of base differences between these sequences is significantly above the PCR error rate (22), but the VHs and Vκs are clearly clonally-related.

The extent of somatic mutation shows that each is clearly derived from a highly mutated progenitor (with 44 consensus substitutions in the VH and 25 in the Vκ). The R:S ratios for the heavy chain gene are moderate, because the number of VH mutations approaches saturation (often 2 or even 3 per codon). Their closest germline counterparts and their clonal relationships are shown in FIG. 10; the pattern is essentially similar for the VH sequences of the AMC-M2 Fabs (not shown). In the genealogical trees for both heavy and light chains, the branching patterns, with variable numbers of successive mutations separating each sequence, are clear evidence of further antigen-driven clonal proliferation and somatic mutation; moreover exach stems from an already-mutated progenitor. Notably also, since neither this heavy nor this light chain was found with other partners, they could both be derived from the same progenitor B cell.

Example 11 Convergent Mutations Suggesting Common Fetal AChR-Specific Selection Processes

We saw recurring replacements in the three independent fetal AChR-specific VH3 genes and especially in the Vκ 02/12 sequences. In brief, there was a CDR1 ³¹S→T substitution in all VH3 Fabs from AMC-M2 and AMC-M6. Even more strikingly, among the Vκ 02/12 light chains, AMC-M2 13K and the majority of the AMC-M6 Fabs not only use Jκ4 but also have common ²²T→S, ²⁷Q→E, ²⁸S→T and ⁵³S→T replacements. The former three contribute to the CDR1 to form a ²²SRASET²⁸ motif that is found in only two other human κ sequences in GenBank. Moreover, these recurring H and L chain mutations were not seen in another fetal AChR-specific Fab from a non-parous EOMG female (Farrar et al 1997) or in the very different AChR β-specific Fab-8 from AMC-M6 (data not shown). 

1) An immunotoxin directed at fetal AChR. 2) The immunotoxin of claim further comprises a human Fab fragment based on a human autoantibody or human combinatorial cDNA library. 3) The immunotoxin of claim 1, wherein said immunotoxin is a pseudomonas exotoxin A-based single chain Fv IT (35-scFV-ETA). 4) The immunotoxin of claim 1, wherein the human Fab fragment is derived from a thymus cDNA library of a Myasthenia Gravis patient. 5) The immunotoxin of claim 1, wherein said immunotoxin is specific for AChRγ subunit. 6) A method of treating a patient with soft tissue tumour comprising the step of exposing the patient to an immunotoxin according to claim
 1. 7) The method of claim 6, wherein said soft tissue tumour comprises nicotinic AChR. 8) The method of claim 6, wherein said soft tissue tumour comprises fetal AChR. 9) A method of treating a patient with sarcoma comprising the step of exposing the patient to an immunotoxin according to claim
 1. 10) The method of claim 7, wherein said sarcoma is Rhabdomyosarcoma (RMS). 11) A method of killing cells comprising the step of contacting the cells with an immunotoxin according to claim
 1. 12) The method of claim 11, wherein said cells are RMS cells. 13) The method of claim 11, wherein said cells comprising nicotinic AChR. 14) The method of claim 11, wherein said cells comprising fetal AChR 15) The method of claim 11, wherein prior to said step of contacting the cells with immunotoxin according to claim 1, the cells are contacted with a chemotherapeutic agent. 16) A method of delaying the development of RMS in a patient comprising the step of exposing the patient to the immunotoxin of claim
 1. 17) A method of delaying the development of RMS cells, comprising the step of contacting the RMS cells with an immunotoxin according to claim
 1. 18) A pharmaceutical composition containing the immunotoxin of claim
 1. 19) A method according to claim 15, wherein said chemotherapeutic agent is selected from the group consisting of a DNA-interactive agent, alkylating agent, antimetabolite, tubulin-interactive agent, hormonal agent, Asparaginase and hydroxyurea. 20) A method according to claim 15, wherein said chemotherapeutic agent is selected from the group consisting of Asparaginase, hydroxyurea, Cisplatin, Cyclophosphamide, Altretamine, Bleomycin, Dactinomycin, Doxorubicin, Etoposide, Teniposide, paclitaxel, cytoxan, 2-methoxycarbonylaminobenzimidazole carbamate and Plicamycin. 21) A method of inducing immune tolerance in a recipient at risk of developing Myasthenia Gravis comprising the step of exposing the recipient to tolerogenic amount of fetal AChR, thereby inducing immune tolerance in a recipient at risk of developing Myasthenia Gravis. 22) A method of inducing immune tolerance in a recipient at risk of developing arthrogryposis multiplex congenita comprising the step of exposing the recipient to tolerogenic amount of fetal AChR, thereby inducing immune tolerance in a recipient at risk of developing arthrogryposis multiplex congenita. 23) A method of preventing Myasthenia Gravis in a recipient at risk of developing Myasthenia Gravis comprising the step of exposing the recipient to tolerogenic amount of fetal AChR, thereby inducing immune tolerance in a recipient at risk of developing Myasthenia Gravis. 24) A method of preventing arthrogryposis multiplex congenita in newborn of mothers with Myasthenia Gravis comprising the step of administering a tolerating amount of fetal AChR to mothers with Myasthenia Gravis or that are at risk of developing Myasthenia Gravis. 